Self-standing NiFe based gas diffusion electrodes toward high-rate AEM water electrolysis

China's ambitious carbon peak and carbon neutrality strategic goals have largely pushed the advances of renewable energy technology. Hydrogen gas is a well-known clean fuel and feedstock in chemical industries, and has gained increased attention in establishing the renewable energy ecosystem. About 90% of the hydrogen gas in China, however, is generated through fossil fuels dependent strategies such as steam reforming. These conventional petrochemical routes contribute to a high carbon footprint and represent unsustainable hydrogen production methods. Low temperature water electrolysis driven by renewable electricity is an alternative way to produce hydrogen and the only byproduct is oxygen, making itself sustainable and carbon footprint free. The commercialized low temperature water electrolysis techniques include alkaline water electrolysis and proton exchange membrane (PEM) water electrolysis. Alkaline water electrolysis (AWE) is a mature electrolysis technology and allows for the non-noble metal-based catalysts. However, its voltage efficiency is insufficient when electrolysis current goes up beyond 400 mA cm(-2). In addition, the long start-up time of AWE cannot match with the intermittent renewable energy. PEM water electrolysis incorporates the design of membrane electrode assembly (MEA) and its voltage efficiency is significantly higher than that of AWE at high-rate conditions (current density above 400 mA cm(-2)), whereas the noble metal catalysts and PEM highly increase the costs of the electrolyzers. Anion exchange membrane (AEM) water electrolysis is a newly developed hydrogen generation technique, and it implements MEA design as well as non-noble metal-based catalysts in alkaline or neutral electrolytes. It thus shows unique advantages in both fast start-up and low costs. Nonetheless, the voltage efficiency of AEM water electrolysis is still not competing with PEM water electrolysis at high-rate conditions, and the sluggish kinetics and mass transfer of oxygen evolution reaction (OER) are critical limiting factors. To address the limitations, it is reasonable to both increase the intrinsic activity of the OER catalysts and enhance the mass transfer/bubble escape for improved performances under high- rate electrolysis. NiFe based electrocatalysts have exhibited appealing intrinsic activity due to the synergistic effects between Ni and Fe in optimizing the oxygen intermediates adsorption and bond formation of oxygen molecules. Anchoring intrinsically active NiFe OER catalysts on electrically conductive porous gas diffusion layer to form self-standing gas diffusion electrodes (GDEs) is one effective method to handle the challenges of high-rate OER for AEM water electrolysis, and has attracted much attention in this fast developed field over the past few years. In light of the importance of self-standing NiFe based GDEs and rapid development of AEM water electrolysis, we in this work review the representing major advances of this type of GDEs in recent years. The general background of OER for high-rate AEM water electrolysis including its challenges, activity and stability indicators and NiFe self-standing GDEs is briefly introduced. Following this section, we pay particular attention to different strategies in preparing the NiFe self- standing GDEs for high-rate OER toward AEM water electrolysis such as magnetron sputtering, cathodic electrodeposition, corrosion engineering and hydrothermal method reported so far. These technique routes are compared regarding their unique advantages and the associated NiFe self-standing GDEs prepared by these methods are analyzed regarding the microstructure and activity. Last but not least, we further outline the critical challenges and perspective in the development and operando characterizations of the NiFe self-standing GDEs for high-rate alkaline and pure-water fed AEM water electrolysis. With the review and discussions in this article, we hope it can serve as helpful references for our research community in developing self-standing GDEs for high-rate AEM water electrolysis.